Field of the Invention
Embodiments of the present invention are directed to methods of making carbon fiber from asphaltenes. In more detail, embodiments of the present invention are directed to methods of making carbon fiber from asphaltenes obtained from heavy oil feedstocks undergoing upgrading in a coking reactor.
Description of Related Art
Carbon fiber comes in several forms and has found wide applicability in military, aerospace, and energy applications primarily because of its extremely high tensile strength per unit weight, anisotropically along the length of the fiber. Carbon fiber also has the significant potential to reduce weight and energy requirements of vehicles, however, it has not to this point been widely adopted in the automotive industry, in part, because of the high cost associated with obtaining and preparing carbon fiber precursors.
Various precursors are currently used to produce carbon fibers of different morphologies and characteristics. Typical precursors include polyacrylonitrile (PAN), cellulosic fibers (e.g., rayon, cotton), petroleum or coal tar pitch, and certain phenolic fibers. Carbon fibers are generally manufactured from these precursors by the controlled pyrolysis of the precursors into fibrous form. The carbon fiber yield will depend on the precursor material. However, a typical PAN precursor will give a carbon fiber yield of about 50% of the mass of the original PAN.
In addition to the precursors listed above, a poorly understood and little-known carbon fiber precursor, asphaltene, may also be used in the production of carbon fibers. Large amounts of asphaltenes are currently being produced from the heavy oils that are mined from heavy oil sand deposits, such as from the Alberta oil sands near Fort McMurray, Canada. In addition to the large amounts of asphaltenes being generated from oil sand deposits, the asphaltenes contained within mined heavy oils typically include hydrogen to carbon ratios from approximately 1:1 to 1.2:1. Such low ratios promote carbon fiber strength and yield because outgassing is reduced during carbon fiber processing.
Asphaltenes are generally comprised of highly-ordered and complex aromatic ring structures typically containing small amounts of hydrogen, nitrogen, oxygen, sulfur, and/or heavy metals in addition to their primary constituent, carbon. They are large, planar, hetero-atom containing molecules that lend themselves to pi-pi bond stacking. Asphaltenes are a distinct chemical component of asphalt, which can typically be isolated from the resins in asphalt or petroleum-based pitch due to their solubility by solvent extraction and other methods. They occur widely in heavy oil-producing formations, and are the non-melting (i.e., burns before melts) solid component of crude oil, giving such crude oils their color. For example, heavier, black-oil crudes, such as those found in tar or oil sands, will typically have a higher asphaltene content.
Asphaltenes are often described and defined based on their solubility. For instance, asphaltenes are soluble in toluene, tetrahydrofuran, and terpenes; however, they are insoluble in n-alkane solvents, such as n-pentane or n-heptane. Thus, asphaltenes may generally be isolated from heavy oils by recovering heavy residuum which is a byproduct left over from heavy oil upgrading. The heavy residuum is generally comprised of oil cuts, asphaltenes, and resin residues. The resins can be separated from the asphaltenes by dissolving the residuum in an n-alkane solvent and filtering out the asphaltenes. To then prepare the asphaltenes as a carbon fiber precursor, the asphaltenes must be dissolved in toluene, tetrahydrofuran, and terpenes, or the like. However, such dissolving and isolating processes are difficult, time-consuming, and expensive. Thus, to this point, isolating asphaltenes from the heavy oils in which they naturally suspended has not been widely utilized to form precursors for carbon fiber production.